Electrodeposition of tin



United States Patent 3,404,075 ELECTRODEPOSITION 0F TIN John Kempton Aiken, London, and Donald Kearey Howard, Manchester, England, assignors to Geigy Chemical Corporation, Ardsley, N.Y., a corporation of New York No Drawing. Continuation-impart of application Ser. No. 269,761, Apr. 1, 1963. This application May 31, 1967, Ser. No. 642,305 Claims priority, application Great Britain, Apr. 5, 1962, 13,075/ 62 10 Claims. (Cl. 204--54) ABSTRACT OF THE DISCLOSURE An additive is used to produce bright tin deposits. The additive is made by sulphonating a phenolic material such as phenol or cresol and thereafter refluxing the product thus obtained for at least one hour at a pressure of 5 to 25 mm. Hg and a temperature of 120 to 190 C.

Cross reference to related applications This application is a continuation-in-part of our patent application Ser. No. 269,761, filed Apr. 1, 1963 now abandoned.

Background of the invention Field of the invention.-The present invention concerns improvements in or relating to tin-plating and in particular toelectroplating compositions for use in the electrodeposition of tin or tin-alloys on metal, and particularly on iron and steel substrates, especially the electrolytic production of tinned sheet, and to electroplating by using such compositions.

Description of the prior art-It is known to use acidic tin-plating baths containing tin in the stannous form, together with small amounts of organic additives. The purpose served by introducing such organic additives into the tin-plating bath is to increase the range of current density over which acceptable plating may be obtained.

More particularly in British Specification No. 659,976 there is described an addition agent for acid tin and tin-alloy electroplating baths consisting of 60 to 85 percent,'by weight, of 4:4'-dihydroxydiphenyl sulphone and the balance essentially 2:4'-dihydroxydiphenyl sulphone, the optimum current density range of acceptable plating being apparently due to synergistic action between the two isomers.

In Canadian Patent No. $80,454, it is stated that chemically pure dihydroxydiphenyl sulphone (whose isomeric constitution is not disclosed) has been found to be inactive in tin-plating baths, and that only the technical grade chemical has the desired activating effect. With the object of producing an improved additive for tin electroplating baths, it has been proposed in Canadian Patent No. 580,454 to modify phenol sulphonic acid by heating the phenol sulphonic acid in the presence of small amounts of phenol and sulphuric acid at a temperature of about 120 C. for about two hours.

Summary of the invention The present invention provides a process of producing an additive for a tin-plating bath which comprises sulphonating phenol and/or cresol and thereafter heating the resulting snlphonated product for at least one hour without adding further phenol or cresol, at a subatmospheric pressure and at a temperature within the range of from 120 to 190 C.

We have found that, surprisingly, the subsequent heat- 3,404,075 Patented Oct. 1, 1968 ing of the product after sulphonation must be effected at subatmospheric pressure in order to obtain additives of satisfactory activity.

The tin-plating additive obtained according to the process of the present invention has been found to be greatly superior to previously-known additives especially in re spect of the current density range over which acceptable plating can be achieved.

The present invention also provides an additive for a tin-plating bath, which additive is produced by the process of the present invention.

The present invention further provides a tin-plating bath comprising a source of tin available for electroplating, and an additive produced by the process of the present invention.

The present invention still further provides an electroplating process which comprises electrodepositing tin using the tin-plating bath of the present invention.

Detailed description of the invention and preferred embodiment thereof More in detail, the present invention provides a process of producing an additive for a tin-plating bath comprising two consecutive stages, namely:

(a) A sulphonation step.

(b) A subsequent step during which the snlphonated product is heated under subatmospheric pressure.

The initial sulphonation step may be conveniently effected by first melting the phenol and/or cresol and adding thereto, at an elevated temperature for instance a temperature within the range of from 60 to C., the sulphonating agent preferably, whilst stirring the mixture. When all sulphonating agent has been added, the sulphonation reaction is preferably taken to completion by heating the reaction mixture at a further elevated temperature for instance a temperature within the range of from to C.

We have found that in order to obtain a product having the best balance of tin-plating activity and solubility in conventional tin-plating baths it is preferred that an ex cess amount of sulphonating agent is employed such that the sulphur trioxide content of the sulphonating agent is about 10% to 30% by weight in excess of the stoichiometric proportion required to sulphonate the phenolic starting-material.

The phenolic starting-materials for use in the process of this invention are advantageously of technical grades. If a mixture of phenol and cresol is used as starting-material, such a mixture preferably contains predominately the cresol component. More preferably however, the phenolic starting-material consists entirely of o-cresol.

In carrying out the sulphonation stage of the process of this invention, chlorosulphonic acid or a commercially available grade of stabilized sulphur trioxide may be used, if desired, but it is particularly advantageous to employ oleum, preferably containing at least 60 percent by weight of sulphur trioxide, as the sulphonating agent.

When the sulphonation step of the process is complete, the subsequent heating of the sulphonated product may be conveniently elfected by slowly raising the temperature of the reaction mixture to a temperature within the preferred range of from 120 to 165 C., and heating the mixture within this temperature range under subatrnospheric pressure for at least one hour. It is particularly preferred When carrying out this post-sulphonation heating procedure to effect the heating at a temperature of about to C., a pressure of about 5 to 24 millimetres of mercury pressure and for a period of about 4 hours or even longer, for instance up to 20 hours. By carrying out the heating of the snlphonated product under subatmospheric pressure, the elimination of water from the sulphonation reaction mixture is facilitated and, moreover, the activity of the final product in tin-plating baths is greatly enhanced.

The precise course of the reaction that occurs during the post-sulphonation heating stage of the process of this invention is not known with certainty, but mixtures of sulphones and sulphone-sulphonic acids appearinter aliato be formed during this stage.

When the post-sulphonation heating stage of this process is complete, the crude reaction product after cooling, may then be advantageously added directly to a tinplating bath in solid form or as an aqueous solution prepared therefrom.

The present invention may be illustrated by, but in no manner limited to, the following examples, in which all parts are by weight.

EXAMPLE 1 150 g. of phenol were melted and gradually reacted with 132 g. of oleum (60%) at 70 C. sulphonation was continued for 1 hour at 100 C. while stirring, and the resultant phenol sulphonic acid was then slowly heated in vacuo to 155160 C. at 11 millimetres of mercury pressure, to keep the distillation of phenol to a minimum. Condensation under vacuum was continued for hours. The yield was 237 g.

EXAMPLE 2 172.3 g. of o-cresol were reacted with 132 g. of oleum 60%) under the conditions of Example 1, the condensation in vacuo being continued for 4 hours. The yield was 257 g.

EXAMPLE 3 Performance trials of typical additives made according to the process of the present invention in comparison with known material A plating bath was made up containing 32.5 g. per litre tin as stannous phenol sulphonate and free phenolsulphonic acid to the extent of 18 g. per litre (calculated as sulphuric acid); 267 ml. portions of this solution were placed in a 300 ml. Hull cell (a known device for plating a cathode at varying current density) and steel cathodes were plated for 1 minute at a total current of 1 amp, the bath temperature being maintained at 42 C. and 1.5 g. of one of the following additives being added in each case:

(1) Pure 4:4'-dihydroxydiphenyl sulphone.

(2) A mixture of 3 parts of purified 4:4'- and 2:4- dihydroxydiphenyl sulphone.

(3) A commercially available product alleged to consist of a mixture as in (2) above.

(4) The product of Example 1.

(5 The product of Example 2.

(6) No additive.

In the table below, measurements are given of the width of the band of acceptable tin-plating shown on the respective cathodes. By acceptable tin-plating is meant the deposition of an integral semibright tin coating which, on flow-melting, gives a bright tin-plate free from pin holes or orange-peeling and of good adhesion.

TABLE Distance of commence- N o acceptable plating observed 4 EXAMPLE 4 258.5 g. of o-cresol were melted and gradually reacted with 248 g. of oleum at a temperature below C. When addition of the oleum was complete the temperature was raised to at which the sulphonation was'continued for 1 hour whilst stirring. The sulphonation product thus obtained was then slowly heated in vacuo'to '150 C. to avoid too rapid water removal; and the vacuum was increased to approximately 5 millimetres of mercury pressure distillation of the stirred mixture being continued for 8 hours. The reaction product had a reddish resinous appearance and a setting point of about 100 C.; it possessed an acid reaction and was somewhat hygroscopic. The yield was approximately 500 g.

When tested in the Hull cell as described in Example 3, save that a concentration of 2 g. of additive per litre of plating solution was used, the following results were obtained:

Centimetres Distance of commencement of acceptable plating from high-current density end of cathode 0.0 Width of acceptable plating band 7.3

On a concentration basis, this material is, therefore, almost three times as active as the most potent additives noted in the table annexed to Example 3. If m-cresol or p-cresol is substituted for o-cresol in the process of this example, good additives are also obtained.

For the purposes of comparison, a sample of modifie phenol sulphonic acid was made according to the following procedure.

900 pounds of phenol was melted and charged into a glass lined kettle. 900 pounds of 20% oleum was then run in slowly so that the temperature did not rise above 200 F. When all the oleum had been run in, the reaction mass was maintained at 200 F. for 45 minutes and then allowed to cool.

The product was heated under reflux for two hours. It was then cooled and diluted to 65% with water.

A plating solution for a continuous plating machine was then made up by dissolving the following ingredients in water:

Grams/litre 65 phenol sulphonic produced as above 60 Stannous sulphate 60 Band width 1 amp position-1.9 0.7 3 amp position5.7 0.8

When similar plating baths were made up but using unmodified, commercially available phenol sulphonic acid as a 65 aqueous solution instead of the modified phenol sulphonic acid obtained as described above, and containing 2.0 grams per litre of plating solution of the product of Example 1 or Example 4 of the instant application, the following results were noted:

1 am 3 am Additive p p Position Band Position Band width width Product example 1...... 0. 0 6.7 4. 9 3. 2 Product example 4 0. 5 7. 5 5. 2 4. 6

These results demonstrate the superiority of the additives of the instant application compared with similar products, previously described.

5 EXAMPLE 5 233 grams of o-cresol were melted and reacted with 224 grams of 65% oleum, the internal temperature of the exothermic reaction being maintained at 60 to 70 C. by means of external cooling. Sulphonation was continued at 100 C. for one hour, while stirring, and the resultant cresol sulphonic acid was then slowly heated in vac-uo to 135 C. at 5 millimetres of mercury pressure. Heating in vacuo was continued for 20 hours. The yield of crude product was 430 grams.

Under the conditions of the standard trials described under Example 6, the crude product of this example gave the following results.

Distance of com- 258.5 grams of o-cresol were sulphonated with 284 grams of oleum (65%) under the conditions described in Example 4.

The sulphonated product was then heated for six hours at 135 C. in the following consecutive stages:

(a) For a period of four hours at a pressure of 5 millimetres of mercury.

(b) For a subsequent period of one hour at atmospheric pressure.

(c) For a further period of one hour at a pressure of 5 millimetres of mercury.

Samples were withdrawn from the reaction mixture at three, four, five and six hours after commencement of the post-sulphonation heating. The plating activity of the four samples under the test conditions set out in Example 3 are noted below:

Sample Time Period Position Band width (hours) (centimetres) (centimetres) prises having present in said bath, in an amount sufiicient to give a bright tin deposit, an additive produced by the steps of (1) sulphonating a phenolic starting material selected from the group consisting of phenol, cresols and mixtures thereof and (2) heating the resulting sulphonated product for at least one hour, without adding further phenol or cresol, at a pressure of from 5 to 25 mm. Hg and at a temperature within the range of 120 to 190 C.

2. The improvement according to claim 1 in which the sulphonating step is effected using oleum containing at least by weight of sulphur trioxide.

3. The improvement according to claim 2 in which the oleum is present in excess of about 10 to 30 percent by Weight above the amount stoichiometrically required in the sulphonation to the respective sulphonic acid, calculated on the weight of the phenolic starting material.

4. The improvement according to claim 1 in which the phenolic starting material is o-cresol.

5. The improvement according to claim 1 in which the sulphonation step is elfected at a temperature not exceeding about C.

6. The improvement according to claim 1 in which the sulphonated product is heated at a temperature of [from about to 7. An aqueous acidic tin-plating bath comprising a source of tin for electroplating and an additive, in an amount sufl'icient to give a bright tin electrodeposit, produced by the steps of (1) sulphonating a phenolic starting material selected from the group consisting of phenol, cresols and mixtures thereof and (2) heating the resulting sulphonated product for at least one hour, without adding further phenol or cresol, at a pressure of from 5 to 25 mm. Hg and at a temperature within the range of 120 to C.

8. A tin-plating bath according to claim 7 in which the sulphonating step is elfected using oleum containing at least 60% by weight of sulphur trioxide.

9. A tin-plating bath according to claim 8 in which the oleum is present in an excess of about 10 to 30 percent by weight above the amount stoichiometrically required in the sulphonating to the respective sulphonic acid, calculated on the weight of the phenolic starting material.

10. A tin-plating bath according to claim 7 in which the phenolic starting material is o-cresol.

References Cited UNITED STATES PATENTS 2,313,371 3/1943 Stack 204---54 2,716,098 8/ 1955 Biedermann 26049 FOREIGN PATENTS 580,454 7/ 1959 Canada. 683,084 11/ 1952 Great Britain.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner, 

